1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive element. In particular, the present invention relates to a piezoelectric/electrostrictive element of the uni-morph type to be used, for example, for actuators, filters, displays, transformers, microphones, sounding bodies (such as speakers), various vibrators, resonators, oscillators, discriminators, gyroscopes, and sensors. The element referred to herein includes elements which convert electric energy into mechanical energy, i.e., into mechanical displacement, force, or vibration, as well as elements which perform conversion reversely from the latter to the former.
2. Description of the Related Art
Recently, it has been demanded, in the fields of optics, precision manufacturing, etc., to use a displacement element for adjusting the optical path length or the position on the order of submicron, and a detecting element for detecting minute displacement after converting it into an electric variation.
In order to respond to such a demand, there have been developed piezoelectric/electrostrictive elements to be used for actuators, which utilize occurrence of displacement based on the inverse piezoelectric effect caused when an electric field is applied to a piezoelectric material such as a ferroelectric substance, or for sensors which utilize a phenomenon (piezoelectric effect) reverse to the foregoing.
For example, the unimorph type piezoelectric/electrostrictive element structure has been widely used in speakers.
The applicant has also previously used piezoelectric/electrostrictive film-type elements made of ceramics in various applications, as described, for example, in Japanese Laid-Open Patent Publication Nos. 3-128681 and 5-49270.
As shown in FIG. 30, the previously proposed piezoelectric/electrostrictive film-type elements have the following structure. Namely, the element comprises a ceramic substrate 104 having at least one window (hollow space) 100 and including a thin-walled vibrating section 102 provided integrally to cover and close the window 100 so that at least one thin-walled wall section (i.e., vibrating section 102) is formed. The element further includes, on an outer surface of the vibrating section 102 of the ceramic substrate 104, a piezoelectric/electrostrictive operating section 112 comprising a combination of a lower electrode 106, a piezoelectric/electrostrictive layer 108, and an upper electrode 110, in which the piezoelectric/electrostrictive operating section 112 is integrally stacked and formed in accordance with a film-forming method. The portion of the ceramic substrate 104 other than the hollow space 100 functions as a fixed section 114 for supporting the vibrating section 102.
The previously proposed piezoelectric/electrostrictive element serves as a compact and inexpensive electromechanical conversion element with high reliability. The known piezoelectric/electrostrictive element provides a large displacement at a low driving voltage. Moreover, the response speed is quick, and the generated force is large. It is acknowledged that such a piezoelectric/electrostrictive element is useful as a constituent component of actuators, filters, displays, and sensors.
However, this piezoelectric/electrostrictive element has a so-called sandwich structure in which the piezoelectric/electrostrictive operating section 112 has an upper electrode 110 and the lower electrode 106 formed on the piezoelectric/electrostrictive layer 108. This piezoelectric/electrostrictive element comprises a piezoelectric/electrostrictive operating section 112 having the sandwich structure, a vibrating section 102, and a fixed section 114, that results in a bending displacement characteristic as shown in FIG. 31B. Namely, the bending displacement characteristic is symmetrical in positive and negative directions of the electric field in relation to a reference electric field point (point of the electric field=0) as a center.
It is assumed that the direction of the bending displacement is positive when the main piezoelectric/electrostrictive element is displaced in a convex manner in a first direction (direction for the upper electrode 110 formed on the piezoelectric/electrostrictive layer 108 to face the free space), while the direction of the bending displacement is negative when the main piezoelectric/electrostrictive element is displaced in a concave manner.
The displacement characteristic is obtained by observing the displacement of the main piezoelectric/electrostrictive element as follows. Namely, the piezoelectric/electrostrictive layer 108 is subjected to a polarization treatment by applying a predetermined voltage between the upper electrode 110 and the lower electrode 106. After that, the voltage applied between the upper electrode 110 and the lower electrode 106 is continuously changed so that the electric field applied to the piezoelectric/electrostrictive element changes, for example, to be electric fields of +3E.fwdarw.-3E.fwdarw.+3E.
At first, an electric field for polarization (for example, +5E) is applied in the positive direction to the piezoelectric/electrostrictive element to perform the polarization treatment for the piezoelectric/electrostrictive layer 108. After that, the voltage application between the upper electrode 110 and the lower electrode 106 is stopped to give a no-voltage-loaded state. Simultaneously with the start of measurement, a sine wave having a frequency of 1 Hz and peak values of .+-.3E (see FIG. 31A) is applied to the piezoelectric/electrostrictive element. During this process, the displacement amount is continuously measured at respective points (Point A to Point D) by using a laser displacement meter. FIG. 31B shows a characteristic curve obtained by plotting results of the measurement on a graph of electric field-bending displacement. As indicated by arrows in FIG. 31B, the displacement amount of the bending displacement continuously changes in accordance with continuous increase and decrease in electric field.
Specifically, it is assumed that the measurement is started from an electric field +3E. At first, as shown in FIG. 32A, the electric field is applied to the piezoelectric/electrostrictive element in the same direction as that of the polarization direction. Accordingly, the piezoelectric/electrostrictive layer 108 is elongated in a direction across the upper electrode 110 and the lower electrode 106, and it is contracted in a direction parallel to the upper electrode 110 and the lower electrode 106. As a result, the entire piezoelectric/electrostrictive element is displaced in the negative direction in an amount of about 0.9 .DELTA.y.
After that, when the electric field is changed from +3E to -0.5E, the displacement amount is gradually decreased. When the electric field is in the negative direction, as shown in FIG. 32B, the electric field is applied in the direction opposite to the polarization direction. Therefore, elongation occurs in the piezoelectric/electrostrictive layer 108 in the direction parallel to the upper electrode 110 and the lower electrode 106, and the displacement is changed to the positive direction.
Next, when the electric field is changed in a direction of -0.5E.fwdarw.-3E, the polarization direction is gradually inverted. Namely, the polarization direction is gradually aligned with the direction of the electric field. As for Point B.fwdarw.Point c.fwdarw.Point C in FIG. 31B, it is assumed that the polarization is inverted approximately completely at Point c, because no hysteresis is observed between Point c and Point C.
As shown in FIG. 33A, the alignment of the polarization direction with the direction of the electric field allows the piezoelectric/electrostrictive layer 108 to change from the state of horizontal elongation to a state of contraction. At a stage at which the electric field is -3E, the displacement amount is approximately the same as the displacement amount (0.9 .DELTA.y) obtained at the start point of the measurement.
When the polarization direction is coincident with the direction of the electric field, the piezoelectric/electrostrictive layer 108 is contracted in the direction parallel to the electrodes 110, 106 (elongated in the direction across the electrodes 110, 106). This situation corresponds to the states represented by Point A and Point C. When the polarization direction is opposite to the direction of the electric field, the piezoelectric/electrostrictive layer 108 is elongated in the direction parallel to the electrodes 110, 106 (contracted in the direction across the electrodes 110, 106). This situation corresponds to the states represented by Point B and Point D. It is noted that there are given 1E=about 1.7 kV/mm and 1 .DELTA.y=about 1.6 .mu.m.
After that, when the electric field is changed from -3E to +0.5E, the displacement amount is gradually decreased. When the electric field is in the positive direction, as shown in FIG. 33B, the electric field is applied in the direction opposite to the polarization direction. Accordingly, elongation occurs in the piezoelectric/electrostrictive layer 108 in the direction parallel to the upper electrode 110 and the lower electrode 106, and the displacement is changed to the positive direction.
When the electric field is changed in a direction of +0.5E.fwdarw.+3E, the polarization direction is gradually inverted. When the polarization direction is aligned with the direction of the electric field, the piezoelectric/electrostrictive layer 108 is changed from the state of horizontal elongation to a state of contraction.
As described above, in the case of the conventional piezoelectric/electrostrictive element, the bending displacement characteristic is symmetrical in the positive and negative directions in relation to the reference electric field point (electric field E=0) as the center. Therefore, the relative displacement amount is small between the no-voltage-loaded state and the voltage-applied state, and the relative displacement amount is small between the states in which mutually opposite electric fields are applied respectively. It is feared, for example, that the displacement amount may be small when the element is utilized for actuators based on the inverse piezoelectric effect, and the sensitivity may be low when the element is utilized for sensors based on the piezoelectric effect.
Therefore, there is a possibility that when the piezoelectric/electrostrictive element is utilized for actuators, the control thereof may become difficult. When the piezoelectric/electrostrictive element is utilized for sensors, it is necessary to connect, at a downstream position thereof, an expensive amplifier having a large amplification factor provided with a considered noise suppression performance.
It is feared that the problems as described above may be extremely disadvantageous especially in the case of production of electronic instruments in each of which a large number of main piezoelectric/electrostrictive elements are used, and it is necessary to maintain a constant quality of each of the piezoelectric/electrostrictive elements, for example, in the case of production of display devices each of which comprises a large number of main piezoelectric/electrostrictive elements arranged corresponding to picture elements (image pixel).